1. Field of the Invention
The invention is generally related to a method and apparatus for thermal cycling assay samples and is specifically directed to a method and apparatus utilizing a constant volumetric fluid flow for thermal cycling assay samples in a sample carrier.
2. Description of the Prior Art
Most prior art thermal cycling systems require variable temperature blocks or require movement of samples between fixed temperature baths. Both of these systems have a number of disadvantages. The block cyclers are inefficient, requiting an undesirable amount of energy and time to operate. For example, the only practical means of increasing ramp speed for thermal cycling a sample in a block configuration is to design the sample tube to maximize its wall exposure to the heat exchanger. While this may accomplish faster ramp times, the basic inefficiencies of the system are not corrected. Further, it is far more desirable to design a more efficient system which will accommodate the many sample tubes already in the field. Therefore, it remains desirable to provide a thermal cycling system that can achieve the required cycle and ramp time without reconfiguring the sample tubes or vials.
The fixed temperature baths are undesirable because of the primary requirement that the samples need to be moved between baths in order to complete the cycle. This also has an undesirable impact on ramp speed variability. Further, in order to eliminate positional temperature variation, it is often required that the sample size be severely limited. For example, when a bath of silicon oil is utilized, the viscosity of the oil reduces convection sufficiently to inhibit adequate heat transfer to the inner samples in an array. This requires that the bath use fewer samples.
In the known prior art, it is often desirable to have a sample array or rack of up to one-hundred sample vials, typically ninety-six. The most commonly used vials are standard 0.5 milliliter microfuge tubes, such as Eppendorf.RTM. brand tubes. In order to meet the positional temperature requirements of a silicon bath using standard vials, it is often required to reduce the sample array by as much as fifty percent of normal sample sizes. In addition, the silicon oil adheres to the sample vials, creating a waste and work environment problem. Without proper removal of the oil from the vials prior to removal of the vials from the bath, the work area can actually become hazardous. The added time involved in properly removing the oil from the vials adds further to the inefficiency of this system.
Other prior art systems include sophisticated equipment to overcome the problems of either the heater blocks or the bath type of thermal cycling systems. For example, as shown in U.S. Pat. No. 4,706,736, entitled "Multi-Zone Heater Arrangement for Controlling the Temperature of a Flowing Medium", issued to S. A. Guiori on Nov. 17, 1987, a multi-zone heater arrangement controls the temperature of a gaseous medium such as air. As the air moves through the multi-zone heater, it is heated to selected temperatures, as required, to provide even heating of the air for various uses. Another mechanism for controlling the flow and temperature of an air stream is illustrated in U.S. Pat. No. 4,868,122 entitled "Arrangement for Drawing Permanent Forms of Microorganisms" issued to J. Kominek, et al., on Sep. 19, 1989. Both of these devices deal more with the even heating and distribution of a fluid or gaseous medium such as air, rather than with the rapid ramp cycling of the medium.
U.S. Pat. No. 4,963,499 entitled "Method for the Calorimetry of Chemical Processes", issued to G. Stockton, et al., on Oct. 16, 1990, discloses a calorimeter for measuring the thermodynamic and kinetic characteristics of chemical reactions and deals more specifically with the reaction of the sample to heating and cooling cycles. This patent is not specifically directed to a method and apparatus for providing rapid, accurate thermal cycling of assays samples.
U.S. Pat. No. 4,981,801 entitled "Automatic Cycling Reaction Apparatus and Automatic Analyzing Apparatus Using the Same", issued to Y. Suzukai, et al., on Jan. 1, 1991, discloses a mechanism for carrying out an enzymatic cycling reaction, including a turntable arranged in a reaction tank, with a number of reaction vessels being arranged on the turntable. The device discloses an antifreeze liquid circulatable through the reaction tank, a heater for heating the antifreeze liquid and a refrigerator for cooling same. A switching valve selectively passes the antifreeze through either the heater or the cooler. The temperature of the antifreeze liquid is controlled to create an enzymatic cycling reaction temperature so as to perform the enzymatic cycling reaction simultaneously for all the liquids contained in all of the reaction vessels for a desired period. This device discloses a mechanism for controlling the temperature of the fluids which is in direct contact with the reaction vessels. However, the cycle time is limited by the ability to heat and cool the fluid as it is circulated through the system.
All of the prior art devices rely on heating and cooling cycles to cool the bath or fluid environment in which the vessels are contained. Thus, the ramp time is naturally controlled by the ability to heat and cool the bath fluids, as well as the vessels and transfer lines containing the fluids. When such systems are used for certain types of nucleic acid cycling, fast ramp speeds can only be achieved by increasing the capacity of the heaters and coolers associated with the fluids, greatly increasing the costs and reducing the efficiency of operation. Typically, faster ramp time requires an inefficient high energy heater, since response time is valued more than energy issues. In such cases, high power systems are utilized to alter temperature in a quick response cycle. On the other hand, highly efficient systems generally have slow intrinsic ramp speeds since the temperature changes are controlled using a low energy level.